Osteoarthritis (OA) is the most common chronic illness of late life and is especially disabling when it affects the hips or knees. Identifying experiences in patients'daily lives that increase or diminish the general severity of OA over time is likely to benefit many older adults. Although daily support from the spouse is likely to impact patients'functioning (pain, stiffness, physical activity), spousal support in relation to physical functioning has been largely overlooked. In the proposed study we will investigate the extent to which daily patient-spouse concordance (agreement) regarding patient symptom severity, arthritis self-efficacy, and symptom catastrophizing is associated with positive spousal support (empathic and autonomy-supportive responses) and problematic spousal support (insensitive and solicitous responses). We will also investigate patients'illness cognitions (self-efficacy, catastrophizing) as means through which spousal support is associated with patients'daily functioning and general OA severity over time. Further, we will examine how spousal support may differentially affect female and male patients'illness cognitions and functioning. This dyadic study will focus on 150 older adults with hip or knee OA (75 male and 75 female), and their spouses. We will combine two 21-day electronic diary assessments and actigraphy (an objective measure of patients'physical activity) with in-person interviews at baseline, 6 months, and 12 months after diary assessment and actigraphy. Our specific aims are: 1) to examine daily patient-spouse concordance and its effects on the quality of spousal support;2) to examine patients'daily illness cognitions as mechanisms that link spousal support with patients'functioning;and 3) to examine the influence of gender on individual and dyadic processes. The study design will enable us to examine variability within and between persons, and the effects of both types of variability on short- and long-term change in OA severity. Findings will have direct implications for strengthening the effectiveness of dyadic psychosocial interventions for OA.